A standard three-part can of the type used to package food is made of a normally cylindrical sleeve and two end caps. The sleeve is manufactured from a rectangular blank of tin plate, that is thin sheet iron or steel coated with tin, that is rolled up with its ends overlapping, and then these ends are seamed together. It is also standard to coat the inside at least of the can with an organic varnish that separates the metallic tin coating from the can's contents, something that is particularly necessary with foodstuffs. This varnish coating is not done at the seam, since it is necessary to get good direct metal-to-metal contact at the overlapping ends that are fastened together to form this seam.
As metallic tin is very expensive it is invariably alloyed with lead to use as solder in making the sleeve seam. Lead is a poison so that it is necessary to coat this seam to separate it from any foodstuffs in the resultant can, even in cans where only a tiny amount of the solder is used at a folded-over seam. Hence a procedure has developed of electric welding the seam of the sleeve, a process which also can leave some of the lead-containing solder exposed on the inside of the sleeve. Recourse is hence had to an organic varnish coating on this seam once it is formed, a process known as striping.
Spraying the coating on often results in a coating that is too thin and like all such spraying operations releases solvent that cannot be left on the product and that must be removed from the site and kept away from the workers. Simply brushing or rolling on the varnish again produces a coating that is too thin, even when several passes are made, and also requires the use of a liquid solvent with the attendant problems. Electrostatically applying the coating has the advantage of being able to produce a thick coating at the exact region that is uncoated, but this process is extremely hard to control. Thus in the food industry where quality control is very strict such a process cannot be used.